Apparatus consisting of an extruder for extruding a thermoplastic material, a sheet die connected thereto and a smoothing roll mechanism to receive an extruded web between at least two smoothing rolls forming an intermediary slit is commonly used for the extrusion of plastic films and their smoothing on both sides. To ensure a uniform thickness throughout the plastic web when plastic sheets and plastic films of more than 0.5 mm in thickness are produced, it is recommendable to keep a small "stock" of the plastic melt in the shape of a bead in front of the intermediary slit being continually pressed out. By this, the intermediary slit is kept fully charged all the time. The web surfaces formed are already completely smooth while cooling and freezing in the intermediary slit, whereas the core of the web does not freeze before it has passed the intermediary slit and with progressive cooling. By controlling the number of revolutions of the extruder roll mechanism one can ensure that the bead is retained at a substantially constant size for a longer time. Thin film webs, however, already freeze throughout their complete thickness in the narrowest area of the intermediary slit and are no longer thermoplastically mouldable. The bead of moulding material in front of the intermediary slit has to be kept to a minimum size. Smoothing of the film surface is only achievable by exerting high clamping forces which lead to a deflection of the rolls. While this deflection can be equalized in advance by accordingly knobbed rolls, such equalization will only be effective at one special operating point. Outside this operating point differences in the slit width between centre and marginal areas are inevitable, entailing corresponding differences in the resulting film thickness.
Increases in the bead thickness induce extremely high pressures at the slit. Variations in the slit width and the pressures appearing there are caused by inevitable deviations in the roundness of the rolls. Local thickening in the extruded web caused by the extrusion process may lead to such high contact pressures locally that the roil surfaces are permanently deformed. To avoid these problems, rubber rolls partly coated with a lacquer finish are used as counter rolls of the steel smoothing roll. The lacquer finish being very sensitive, however, these rolls are normally of short longevity.
Controlling of extrusion and smoothing of thin plastic films thus requires a careful adherence to constant production conditions and especially the use of film dies having adjustable outlet lips, socalled "flex lips" Nevertheless there are often deviations or troubles in practice which generally cause damages to the roll surface resulting from extreme pressures in the smoothing slit.